Many highly stressed fastener joints are used today, especially in the aerospace industry. Such fastener joints generally use a fastener with a head at one end, a bearing section which can be extended through the holes in the work pieces integral with the head and an engagement section at the opposite end of the bearing section which is designed to be engaged by a locking device such as a nut or collar to maintain the fastener in position. Two important criteria are usually followed in such fastener joints, the first being the maintenance of good bearing contact between the fastener and the work pieces and the second being the keeping of the weight of the fastener joint at a minimum.
Generally, two types of fasteners are used in these fastener joints. The first type of fastener is a fastener having a substantially constant diameter bearing section that is placed through substantially constant diameter holes in the work pieces and the second type is a fastener with a tapered bearing section that tapers outwardly from the engagement section toward the head which is placed in a correspondingly tapered hole through the work pieces. While tapered fasteners such as those disclosed in U.S. Pat. Nos. 3,034,611 and 3,304,109 do produce fastener joints with good bearing contact between the bearing section of the fastener and the work pieces, the major difficulty with using such fastener systems is that it is extremely difficult to produce the correct tapered hole through the work pieces. It will also be noted that such tapered fasteners tend to be forced out of the holes, head end first, thereby requiring a strong locking device such as a nut or collar to retain the fastener in place. When using a constant diameter bearing section fastener system, the hole forming process is considerably simplified, however, it is more difficult to obtain good bearing between the bearing section of the fastener and the work pieces. The requirement of these prior art systems of a locking device such as a nut or collar has resulted in the systems remaining relatively heavy thereby reducing the overall pay load of the structure in which they are used.
Techniques have also been developed for increasing the fatigue life of the joint. One of these techniques is commonly known as coldworking wherein the hole is expanded to such an extent that the metal immediately surrounding the holes where the localized expansion occurs is stressed beyond its compressive yield point and after which the hole is permitted to return or rebound toward its original diameter to establish a compressive stress gradient immediately surrounding the hole. Such coldworking techniques are disclosed in U.S. Pat. Nos. 3,434,327; 3,566,662; and 3,805,578. Another technique which has been used in this regard is commonly known as an interference fit where the diameter of the bearing section of the fastener is greater than the nominal diameter of the holes in which the fastener is fitted. The techniques of coldworking and interference fit have been combined in special fasteners such as disclosed in U.S. Pat. Nos. 3,578,267 and 3,779,127. It will be seen that the fasteners used in these techniques also require the locking device such as a nut or collar thereon to retain the fastener in place.